The present invention relates generally to the field of oscillators, and in particular, to oscillator/buffer amplifier circuits which exhibit low power consumption and low harmonic output.
Crystal controlled oscillators, such as Colpitts oscillators and the like, are commonly used to provide a highly stable reference frequency for a wide variety of radio communications equipment. These reference oscillators must conform to strict performance specifications including, among others, temperature stability, aging, load isolation, frequency adjustment range, and output distortion. Portable radio applications, however, further impose the significant requirement of low power consumption. It is very important that all portable radio circuitry, including reference oscillators, be optimized for minimum current consumption to extend battery life. Furthermore, portable applications present severe size and cost limitations to the oscillator design. Typical prior art crystal oscillators comply with the frequency stability, temperature stability, frequency adjustment range, and load isolation criteria, yet fail to address this need for a simple, inexpensive, low power reference oscillator with reduced harmonic content.
Nugent et al., for example, in U.S. Pat. No. 3,832,653 entitled "Low Noise RF Signal Generator" incorporates three additional inductor-capacitor (L-C) filter networks into a Colpitts oscillator/common-base (C-B) buffer circuit to reduce oscillator noise and suppress harmonic output. This method of adding supplementary L-C filter components to the output buffer circuitry precludes usage in portable radio applications, since the utilization of each supplementary component introduces the issue of reliability, as well as contradicts the aforementioned size-minimization and cost-reduction goals.
Minch, however, in U.S. Pat. No. 3,958,190 entitled "Low Harmonic Crystal Oscillator" incorporates the common-base buffer stage directly into the oscillator tank circuit. The oscillator crystal itself acts as a very narrow bandpass filter to the currents in the tank circuit. Since the emitter-base junction of the C-B buffer stage is now included as part of the tank, and seeing that the C-B buffer transistor is biased to operate linearly, very few harmonics are coupled to the output. This approach complies with the size and cost restrictions of portable applications, because the supplementary L-C filter components in the output stage are no longer necessary. Nevertheless, the problem of increased power dissipation remains, due to the fact that the output buffer stage approximately doubles the current drain of the oscillator circuit itself.
A need, therefore, exists for a low power oscillator circuit with reduced harmonic output that avoids the additional bulk, expense, complexity, and unreliability of supplementary harmonic filter components.